The present invention relates to protective circuits for use with a boom, such as a crane or aerial platform apparatus boom, provided with a boom-deflection or overload sensing device.
Cranes of the boom type have long been known, these cranes being provided with a boom which is capable of being raised and lowered about a horizontal axis, and swung about a vertical axis. Such booms may also be telescopic, and such booms are also used in connection with aerial platform apparatus, which latter have a workman's platform or basket, usually at the outer end of the boom. As is well known, the load on the boom, tending to cause it to deflect, increases with an increase in the weight carried, with a lowering of the boom from a near vertical position to a horizontal or near-horizontal position, and with telescopic extension of the boom. Each of these movements may be considered as a "forward" movement, whereas the opposite movements, such as reducing the weight, raising the boom and telescopically retracting the boom may be known as rearward movements, it being understood that the forward movements increase the stress on the boom and its components, whereas rearward movements decrease the stress on the boom and its components.
A number of devices have been suggested for detecting or sensing the stress in the boom, and consequently the approach of the loading of the boom to an overload condition. An overload condition is one in which the boom is at or closely approaching a failure condition. In addition, where the boom is a part of a mobile, vehicle-mounted crane or aerial platform apparatus, an overload condition may be deemed as a condition wherein tipping of the machine is being approached during a forward movement of the boom. One known device for sensing boom overload comprises a beam having one end fixed to the boom, and extending generally parallel to the boom, the beam having a free end remote from its fixed end. A micro-switch is provided at or adjacent the beam free end, and is mounted so that it is caused to be actuated to open position upon deflection of the boom beyond a predetermined amount. The micro-switch is, in turn, connected with circuitry which may include conductors in the control circuits of the boom controlling forward movement. Hence, an overload condition which will cause actuation of the micro-switch and thereby break the forward direction control circuits will prevent movement of the boom towards a more dangerous condition, but permit reverse movement, so that the position or condition of the boom may be changed towards a safer condition.
It has been found that with the overload sensor of the beam and switch type, certain problems have arisen in connection with actual operation. Specifically, as the boom is being moved in a forward direction, for example as it is being lowered towards the horizontal, the deflection of the boom which causes actuation of the micro-switch by the beam is not a steady progression, but instead there is some variation in the boom deflection. This is due to the necessary flexibility of the boom, which is believed to be the primary, if not the sole reason for the boom to oscillate so as to give rising and falling values of boom deflection, as the boom continues in the above mentioned forward direction.
This oscillation of the boom has been found to cause the micro-switch to open and close, as the boom passes into a transition zone intermediate between an overload condition zone and a non-overload condition zone. Since the overload sensor switch would thereby be caused to open and close, it would also cause the forward movement control switches to open and close, and there could be built up within the system an ever increasing oscillation of the boom, which would be deletorious. The boom could possibly be thereby caused to destroy itself, or damage itself, or, where there is a workman in a platform or basket, in the case of an aerial platform apparatus, the workman could be injured, as by being thrown from the platform or basket.